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  <title>Document</title>
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  <script>
    function longestIdealString(s, k) {
      const n = s.length;
      const arr = new Array(26).fill(0);// arr[i]为以'abc...z'[i]结尾的子序列最大长度
      for (let i = 0; i < n; i++) {
        const code = s.charCodeAt(i) - 'a'.charCodeAt(0);
        let max = 0;
        // [0,i)区间的子序列结尾的字母如果符合条件，则比较大小，选出最长的加上当前字符长度即可
        for (let j = Math.max(0, code - k); j <= Math.min(25, code + k); j++) {
          max = Math.max(max, arr[j]);
        }
        arr[code] = max + 1;
      }
      return Math.max(...arr);// 从不同字母结尾的字符串中找到最长的
    };
    console.log(longestIdealString(s = "acfgbd", k = 2))
  </script>
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